FeLV is a retrovirus of the sub-family oncovirinae. The virus has a simple genomic organisation with three genes, gag, pol and env, encoding vital proteins.
FeLV occurs in three subgroups, A, B or C, which differ in their env genes. FELV-A is present in every isolate and is efficiently transmitted between cats in nature. Viruses of the other two subgroups arise as variants of FeLV-A in individual cats by recombination of endogenous FeLV env genes (FeLV-B) or mutation within the env gene (FeLV-C). Hence the occurrence of subgroups B or C is dependent upon natural infection of cats with FeLV-A.
FeLV is responsible for several fatal diseases in domestic cats including lymphoid and myeloid leukaemias, fibrosarcomas, anaemia, immunodeficiency, enteritis and reproductive failure. The virus is common in nature: a recent survey in the U.K. indicated that the overall prevalance of infection is approximately 15% of sick cats and 6% in healthy cats. These cats have a persistent infection with viraemia and are a source of infection for susceptible animals. FeLV is transmitted either by contact through the transfer of infectious saliva or prenatally across the placenta.
Following natural or experimental exposure of cats to FeLV, there are two possible outcomes. Cats either develop a persistent, life-long infection or recover. Persistently infected cats have a viraemia while recovered cats have no virus in their blood. Some cats which recover may harbour a latent infection in the bone marrow for several weeks or months before it is eliminated. Recovered cans are immune to FeLV as evidenced by their resistance to subsequent challenge with virus.
Two factors influence the outcome of infection. The first is the age an which a cat is exposed to virus. Kittens up to 14 weeks of age are very susceptible and essentially all can be affected by virus challenge. Older kittens are more resistant so that by 16 weeks of age only about 20% can be infected. The other factor governing susceptibility is dose of virus: high doses lead to persistent viraemia while lower does immunise. The consequences of persistent infection are severe and essentially all viraemic cats die within four years of infection. By contrast recovered cats have a normal life span.
Recovery is due to an immune responsa to the virus. Only antibody responses have been examined in detail. However, it is clear that resistance to reinfection is mediated by virus neutralising antibodies which are directed at the envelope surface glycoprotein the virus, gp70. Attempts have been made to immunise cats against FeLV infection with vaccines containing gp70, some of which have been successful.
European patent specification EP 0377842 describes the production of replication-defective FeLV viral sequences. U.S. Pat. No. 4,794,168 relates to the use of immunogens from FeLV envelope protein P15E in vaccines. European patent specifications 0247904, 0173997 and 0216564, International patent publication WO85/02625 and U.S. Pat. No. 4,789,702 all describe the use of the FeLV envelope protein gp70 or fragments thereof as antigens.